In a mobile communication network, a user equipment in an idle mode uses a random access channel (RACH) to transmit an initial control message to a network. In other words, the user equipment uses the RACH to adjust time synchronization with the network or acquire a radio resource in case of no radio resource in an uplink to which data will be transmitted when the user equipment desires to transmit the data to the uplink.
For example, if the user equipment is powered on and then desires to first access a new cell, the user equipment should transmit an access request message to the uplink for radio resource control connection (RRC connection) after adjusting synchronization of a downlink and receiving system information of a cell which the user equipment desires to access. However, since the user equipment is not adjusted to the current network in time synchronization and does not ensure the radio resource of the uplink, the user equipment requests a base station of a radio resource for transmission of a connection request message to the network through the RACH. The base station which has been requested the radio resource from the user equipment allocates a suitable radio resource to transmit the RRC connection request message to the corresponding user equipment.
For another example, it is assumed that the user equipment is in an RRC connection mode as RRC connection is formed between the user equipment and the network. In this case, the user equipment is allocated with a radio resource depending on radio resource scheduling, and transmits user data to the network through the corresponding radio resource. However, if data to be transmitted does not remain in a buffer of the user equipment, the network does not allocate the radio resource to the corresponding user equipment any more. At this time, the buffer status of the user equipment is reported to the network either periodically or whenever an event occurs. In this case, even though new data occurs in the buffer of the user equipment to which the radio resource is not allocated, since there is no radio resource allocated to the user equipment, the user equipment requests the network to allocate a radio resource required for data transfer, through the RACH.
Hereinafter, an initial access procedure of the user equipment to the network through the aforementioned RACH in a long term evolution (LTE) system will be described, wherein the LTE system has attracted attention as the communication standard for next generation.
FIG. 1 is a signal flow chart illustrating an initial access procedure of the user equipment discussed in the LTE system according to the related art.
The user equipment selects RACH signature and RACH occasion through system information transmitted from the base station, and transmits the selected RACH signature and RACH occasion to the base station through a random access preamble (S101).
After receiving the random access preamble from the user equipment, the base station transmits a random access response to the corresponding preamble to the user equipment (S103). The random access response includes timing offset information (Time Advance: TA) and information of radio resource allocation of the uplink for transmission of the RRC connection request message.
After receiving the random access response, the user equipment transmits the RRC connection request message in accordance with the information of radio resource allocation included in the random access response (S105).
After receiving the RRC connection request message from the user equipment, the base station transmits RRC connection establishment message or RRC contention resolution message to the user equipment depending on circumstances (S107).
In the RRC connection procedure using random access according to the related art, if it is necessary to limit uplink access of the user equipment due to frequent transmission of the RRC connection request message, a wireless network transmits RRC connection rejection message, which includes re-direction information, to allow a specific user equipment to be re-directed to another frequency band or another system. However, since the re-direction information according to the related art is transmitted through the fourth message (S107) of random access, a problem occurs in that the wireless network fails to promptly control uplink access of the specific user equipment.